Young’s modulus degradation in fatigue of filled polymers for household appliances

Contrary to metals, composite materials suffer not only a strength, but also a modulus decrease when subjected to cyclic stresses. Therefore, establishing a correlation between the fatigue parameters and the modulus degradation becomes important in stiffness-critical design. Besides, the knowledge of the law governing the modulus decay would be a powerful tool in predicting the residual fatigue life through simple non-destructive tests. The material tested in this work was polypropylene charged with CaCO3 mineral filler, often employed in household appliances for its attractive properties, such as good mechanical properties coupled with low density, high damping, good acoustic isolation, high chemical stability and low cost. Rectangular specimens were fabricated by injection molding and loaded in simple bending under displacement control at low frequency, to avoid excessive heating. Four different maximum displacement values were selected, and for each the modulus decrease was monitored periodically, to assess its variation with cycle evolution. As expected, the modulus decay was the more evident, for a fixed number of cycles, the larger was the maximum displacement imparted in fatigue. To analytically express the decrease in modulus as a function of cycles elapsed, a simple two-parameter model available from the literature was assumed and suitably modified. From the data generated, the formula was able to describe the modulus variation in fatigue; however, the parameters appearing in it were strongly dependent on the maximum displacement set. To resort to a more general expression for the phenomenon under study, the experimental results were thoroughly analysed: it was found that the dependence of both the constants on the displacement can be modelled by simple linear relationships. By this finding, the fatigue data pertaining to all the fatigue conditions adopted could be reduced to a single master curve.